Logseq is one I highly recommend: http://logseq.com/ Outliner, local file storage, API plugins, todo's and loads more. On top of everything, it is free.
On the roadmap there are plans to introduce a lightweight mobile app in the near future. Here's hoping this will come soon but the Devs are doing a cracking job
I experienced the same when I first started using it. It was a bit unstable and you had to tread with caution. There has been a significant amount of updates and the stability has improved massively. The local storage option also means that there is less risk of something going wrong than with the GH sync - at least I have found it to be a lot smoother.